Amorphous PET material, in other words, PET material which has a relatively low degree of crystallisation, becomes dimensionally unstable at relatively low temperatures. In general, an article of amorphous PET material becomes dimensionally unstable at approximately 70.degree. C., and for safety, it is recommended that such articles should not be subjected to temperatures in excess of 50.degree. C. It is known that by increasing the degree of crystallisation of the PET of the article, the temperature at which the PET becomes dimensionally unstable is increased. By increasing the degree of crystallisation of the PET material of an article above 25%, the article remains dimensionally stable at temperatures in excess of 200.degree. C. Increasing the degree of crystallisation of the PET material of the article further increases the temperature at which the article remains stable.
It is known to thermo-form an article from a sheet of PET material, and in the thermo-forming of the article, provide the article with a degree of crystallisation which increases the temperature at which the article remains dimensionally stable. For example, U.S. Pat. No. 3,496,143 describes a method for thermo-forming an article from a sheet of PET material using a vacuum deep drawing apparatus. The PET sheet is of 3 mm thickness, and of area 370 mm by 280 mm. The method requires that the PET sheet should have an initial degree of crystallisation lying in the range of 5% to 25%. The method requires the PET sheet to be subjected to heat treating prior to forming in the vacuum mould of the apparatus. On being formed, the sheet, while still in the mould, is subjected to further heat treatment by heat transfer from the mould. Heat is transferred into the formed sheet through the surface of the sheet in contact with the mould. The formed sheet is retained in the mould until the degree of crystallisation of the formed sheet is greater than 25%. It is suggested in the specification that the article formed is dimensionally stable at a temperature of 160.degree. C. for sixty minutes. In other words, the moulded article was heated and maintained at a temperature of 160.degree. C. for sixty minutes and it proved to be fully dimensionally stable during the sixty minute period.
It has been found that, in general, the method proposed in U.S. Pat. No. 3,496,143 is unsuitable for thermo-forming a dimensionally stable article from PET sheet material of thickness greater than 1 mm. In particular, it has been found that articles produced using the method proposed in the U.S. specification become dimensionally unstable at temperatures in excess of 80.degree. C. Furthermore, the time taken to increase the degree of crystallisation of the PET sheet above 25% using the method proposed in the U.S. specification is uneconomically long.
There is therefore a need for a method and apparatus for forming an article from a PET sheet which is dimensionally stable at temperatures in excess of 70.degree. C., and in particular, there is a need for a method and apparatus for forming an article from a sheet of PET material of thickness greater than 1 mm.